Embodiments of the present invention relate to a pre-distortion type distortion compensation technology to suppress nonlinear distortion of transmission amplifier output by performing distortion compensation processing on an input signal into a transmission amplifier in advance in radio communication.
Generally, a high-efficiency transmission amplifier used in a radio transmission apparatus such as a mobile base station has strong nonlinear characteristics. Thus, when a modulating signal for high-speed radio communication is transmitted, such nonlinear distortion in the transmission amplifier causes out-of-band radiation power in the transmission modulating signal, affecting adjacent transmission channels.
As a method for suppressing out-of-band radiation by a transmission amplifier, a pre-distortion method is known by which nonlinear distortion in the transmission amplifier is compensated for by adding a distortion signal having characteristics opposite to nonlinear distortion characteristics of the transmission amplifier to an input signal before the input signal is input into the transmission amplifier. Particularly, an adaptive pre-distortion method by which distortion compensation is adaptively made by giving feedback of output of the transmission amplifier to the input side may suppress out-of-band radiation.
FIG. 12 is a diagram of principle of the pre-distortion method. In the transmission amplifier, output is normally saturated with increasing input power so that a linear signal may not be output in response to an input signal (1201 in FIG. 12). Nonlinear characteristics of the amplifier have a harmful influence described below.
That is, nonlinear characteristics of the transmission amplifier cause for amplifier input radiation of unnecessary spectra out of the signal band. The out-of-band radiation power deteriorates characteristics of other systems using out-of-band frequencies. Moreover, unnecessary spectra are radiated in the signal band as well. Such radiation causes characteristic deterioration of the signal itself.
Further, most current digital modulation methods require linear amplification characteristics and thus, when an amplifier having the above saturation characteristics is used, the use of a linear low input power portion is unavoidable. This leads to lower power efficiency of the transmission amplifier.
Therefore, characteristics opposite to amplifier characteristics are applied to an input signal of the transmission amplifier using the pre-distortion technology (1202 in FIG. 12). With the addition of such nonlinear amplifier characteristics, as indicated by 1203 in FIG. 12, compensated linear characteristics may be obtained as resultant transmission amplifier output.
As a method of pre-distortion, a pre-distortion method using power series has been proposed. This is a method, as illustrated in FIG. 13, by which a compensation operation in a pre-distortion unit 1301 prior to the transmission amplifier is performed by a power series operation on an input signal x.
That is, the pre-distortion unit 1301 in FIG. 13 makes compensation for distortion of a transmission amplifier 1305 by performing a power series operation on the input signal x.
Output of the pre-distortion unit 1301 is converted into an analog signal by a D/A converter 1302 and further quadrature-modulated by a quadrature modulator 1303 based on a signal oscillated by a local oscillator 1304 in accordance with a transmission base station.
The modulated transmission analog signal is power-amplified by the transmission amplifier 1305 and output thereof is supplied to a transmission antenna 1307 via a coupler 1306 before transmission from there.
Moreover, feedback of output of the transmission amplifier 1305 is given to the input side from the coupler 1306.
That is, output of the coupler 1306 is down-converted by a down-converter 1308 based on a signal oscillated by a local oscillator 1309 in accordance with a transmission base station. Output thereof is further converted back to a digital signal by an ND converter 1310 before being restored to a base band by a demodulator.
An error signal e(n) of a resultant feedback signal Sfb(n) from a transmission signal Sref(n) delayed by a delay circuit is calculated by a subtracter 1311.
Then, power series operation coefficients a, b, c, and d to be supplied to the pre-distortion unit 1301 are updated by a coefficient update unit 1312 so that the error signal e(n) is minimized based on the least mean square operation.
Power series operation coefficients are gradually converged to predetermined values in this manner and a power series operation on the input signal x is performed by the pre-distortion unit 1301 using the power series operation coefficients that have converged to the predetermined values. Accordingly, in a steady state, nonlinear distortion characteristics in an analog circuit unit are suppressed with precision while high power efficiency is retained. Then, even if the nonlinear distortion characteristics fluctuate due to an influence of temperature or frequency, an analog gain fluctuation amount thereof is detected by the feedback signal Sfb(n). Then, values of power series operation coefficients are updated in a compensating direction of the fluctuation amount by the coefficient update unit 1312 so that characteristic fluctuations may dynamically be compensated for.
The above configuration actually has a configuration for a complex signal.
In the above configuration of conventional technology, it is assumed, for example, that two sine wave signals (2-tone signal) a frequency 2Δf apart from each other represented by the following formula are input into an amplifier model that is modeled by power series:cos 2π(fc−Δf)t+cos 2π(fc+Δf)t, where fc is a carrier frequency.
As a result, even-order power terms of an output signal represented by power series contain only signal components that are significantly detuned from the carrier frequency fc and suppressed by a filter of the analog unit or the transmission amplifier itself. In contrast, unnecessary components arise near the carrier frequency such as fc±3Δf for a tertiary power term and fc±5Δf for a quintic power term. Therefore, nonlinear distortion in the transmission amplifier 1305 may be modeled by power series having only odd-order power terms. Thus, as illustrated in FIG. 13, power series to be processed by the pre-distortion unit 1301 are also generally composed of odd-order power terms only.
Hereinafter, a simple power series formula ax+bx3+cx5+dx7 will be used as a power series formula for the sake of simplicity. For actual distortion compensation, more complex series that take delay components into consideration including Volterra series are generally used to model characteristics of the transmission amplifier 1305 more accurately (see, for example, V. J. Mathews and G. L. Sicuranza: “Polynomial Signal Processing”, John Wiley&Sons, Inc. (2000)).
Generally, electric energy of a transmission signal changes in a transmission apparatus in a radio communication system depending on utilization conditions of the user. In a distortion compensation method by conventional power series described above, there is a problem that out-of-band radiation power temporarily rises unless optimal distortion compensation coefficients are used immediately after power of a transmission signal fluctuates.
If, for example, distortion compensation coefficients are updated in a state in which the peak power of a transmission signal is “peak power 1” in FIG. 14, a difference arises between a correction curve 1403 in a region (thick line) where no signal arises for amplifier characteristics 1401 and an ideal correction curve 1402. If, in this state, the peak power of a transmission signal changes to “peak power 2”, coefficient characteristics 1404 before the signal changes will be used for distortion compensation operation. As a result, correction curves 1403 and 1404 deviate from the ideal correction curve 1402 until the coefficients are updated and thus, there is a problem that adequate performance for distortion compensation may not be maintained.